In communications systems such as a CATV system, the headend is the originating point for a broadband information signal which is broadcast to subscribers. Signal sources input to the headend include over-the-air stations, satellite services, and terrestrial services, such as via microwave relay. In addition, local programming services may also be added to the broadband information signal at the headend. All of these source signals are processed and combined at the headend into an RF broadband information signal for transmission over a distribution network.
At the headend, the RF broadband information signal is converted into light and is modulated onto a light source for transport from the headend as an optical signal beam (hereafter called the "optical signal") via fiber optic cable to a plurality of nodes. At each node, the optical signal is detected by an optical receiver and is converted back into an RF broadband information signal to be provided to subscribers via coaxial cable.
At the headend, an optical transmitter launches the optical signal into a fiber optic cable for transport to subscribers. Typically, the optical signal comprises a plurality of optical signals launched at different, but closely spaced wavelengths, typically around 1550 nanometers (nm) or 1310-nm. For the purposes of this discussion, the term "wavelength" (and its designation ".lambda.") will be used to identify an optical signal having a particular wavelength. Therefore, as used herein, the terms "optical signal" and "wavelength" should be understood to be interchangeable.
In CATV systems, there may be over 100 different wavelengths ranging from 1530 to 1560-nm launched on each fiber optic cable. Wavelengths may be stacked as close as 0.4-nm and each wavelength typically carries between 80-130 RF channels. Some wavelengths include RF channels having frequencies from 50-550 MHz or from 50-1000 MHz. Because each wavelength can carry many different channels, each wavelength can "target" a particular node, such that different wavelengths carrying different RF channels can be provided to different groups of subscribers.
In DWDM fiber optic communications systems, the optical fiber generates nonlinear effects based on launch power, fiber material characteristics, four wave mixing (FWM), and system parameters. Among these nonlinear effects is stimulated Raman scattering (SRS). When multiple optical signal beams having different wavelengths are transported on an optical fiber, SRS causes some of the energy in a lower wavelength to be transferred to the next adjacent higher wavelength. So, for example, if four wavelengths .lambda..sub.1 -.lambda..sub.4 are launched into a fiber, with .lambda..sub.1 being the lowest wavelength and .lambda..sub.4 being the highest, then .lambda..sub.1 would transfer some energy to .lambda..sub.2, .lambda..sub.2 would transfer some energy to .lambda..sub.3, and .lambda..sub.3 would transfer some energy to .lambda..sub.4. In this manner, the energy is "uptilted" toward the higher wavelengths. Also, in analog systems, SRS causes each wavelength to modulate the other wavelengths. This phenomenon is similar to RF cross-modulation and is called SRS induced Raman cross-talk, or nonlinear optical cross-talk. For the purposes of this discussion, this phenomenon will be referred to as "Raman cross-talk".
SRS amplification occurs at high powers, typically 1-2 watts, which are not usually encountered in fiber optic communications systems. However, Raman cross-talk occurs at much lower powers, such as above 3-5 dBm, which makes the Raman cross-talk effect relevant at the low powers found in fiber optic systems. Raman cross-talk degrades the fidelity and picture quality on a subscriber's television set or monitor, and should therefore be minimized.
The magnitude of Raman cross-talk is dependent on several factors, including wavelength spacing, the number of channels, the optical power per wavelength, chromatic dispersion, and fiber length. With regard to the factor of wavelength spacing, Raman cross-talk increases with wavelength spacing up to 120-nm. An important consideration is that Raman cross-talk does not occur between two optical signals that have orthogonal states of polarization.
The polarizations of optical signals launched into an optical fiber are typically described with reference to the principal states of polarization (PSP) axes of an optical fiber. These axes are generally referred to as a fast (vertical) state of polarization and a slow (horizontal) state of polarization, which are orthogonal to each other. For the purposes of this discussion, optical signals having a vertical and horizontal polarity, respectively, are deemed to have orthogonal polarities. Optical signals are deemed to be aligned when both signals have the same polarity, whether it be vertical or horizontal. Therefore, when optical signals are aligned, Raman cross-talk is maximized, but when optical signals have orthogonal polarizations, Raman cross-talk is nulled.
In current DWDM communication systems, optical signals are launched into an optical fiber such that the orientation of the state of polarization of each optical signal is random. Therefore, the polarizations of optical signals launched into an optical fiber are typically not all the same. However, there is a high probability of all polarizations being aligned and a low probability of all polarizations being orthogonal to each other. Therefore, there is a need in the art to reduce the effect of Raman cross-talk that occurs when optical signal beams having different wavelengths are combined for transport via an optical fiber by launching the different optical signals into the fiber with orthogonal polarities. There is also a need to reduce Raman cross-talk by launching optical signals such that any Raman cross-talk that does occur will occur only between closely spaced wavelengths.